The present invention relates to video transmission systems employing sub-band coding and particularly concerns methods and apparatus for reducing block artifacts in the reproduced video image of a system employing complementary sub-band coding and decoding operations.
Sub-band coding is a process which has been frequently proposed for compressing a wideband video signal for transmission through a channel of limited bandwidth, such as a standard 6 MHz television channel. Sub-band coding, as the term is used herein, refers generally to a process wherein a video image signal is converted into a plurality of spectral coefficients representative thereof and may be effected either by spatially filtering the video signal or by subjecting it to a suitable block transform, such as the discrete cosine transform (DCT). In either case, the video signal is separated into a plurality of sub-bands each comprising a series of coefficients with the coefficients derived for each sub-band representing a different spectral component of a respective block of the video image. The sub-band coefficients are subsequently processed in accordance with a data compression algorithm to allow for their transmission within the available bandwidth of the transmission channel. A system for variably quantizing or truncating the coefficients according to an adaptive algorithm is disclosed in the above copending application.
The received coefficients are processed by a complementary decoder to reconstruct a representation of the source video image. Due to the quantization or truncation of the coefficients before transmission, the reconstructed image may be characterized by block artifacts which create a visually perceptible blockiness in the reproduced image. Block artifacts are particularly pronounced along diagonal edges which cut through the block structure of the image. These edges are reproduced with relatively low resolution and actually assume a staircase shape rather than a straight line. This effect, along with other block artifacts, is very perceptible to the viewer and therefore quite undesirable.